In the hobbing process a rotating, generally cylindrical-shaped tool having helically arranged stock removing surfaces is brought into contact with a rotating workpiece, generally a gear blank. In spur gear hobbing, the tool and workpiece rotate in a timed relationship as though the workpiece were a gear rotating in mesh with a worm gear represented by the hobbing tool. When helical gears are hobbed, a supplemental rate of motion is applied to the workpiece rotation, either advancing or retarding the fundamental timing relative to hobbing tool rotational rate, in order to develop the appropriate helix angle across the face width of the gear teeth being machined. Hobbing is primarily used for producing spur and helical gears; however, other products, such as worm wheels, sprockets, and splined shafts may also be produced by the hobbing process.
In the process of hobbing spur and helical gear teeth, splines, and sprockets, a relatively short lengthwise region on the hob is employed in the generation of teeth on a given workpiece. While it would be possible to use very short hobs in general on hobbing machines (with the possible exception of those used for worm wheel hobbing), without ever shifting the tool lengthwise along its axis of rotation relative to the workpiece, in practice this is an uneconomical approach, because of the frequent tool exchange interval that would result. Such hobs are used only as a last resort under unusual conditions in which any relative shifting would result in mutilation of adjacent portions of the workpiece.
The general practice is to use a relatively long cylindrical hob, which is shifted along its axis either periodically in small increments during intervals between cutting operations, or continuously during the cutting operation in order to allow all regions along its length to cut and thus spread the tool wear over the entire tool. An adjustment in the workpiece rotational rate is made when continuously shifting to maintain timing with the tool. The purpose of axially shifting the tool, in either case, is simply to avoid excessive wear at any given location on the hob, thus maximizing the interval between tool exchanges and minimizing the amount of stock removed when the tool is resharpened.
Over the years there have been many approaches employed for shifting the hobbing tool along its axis relative to the work gear. One very common approach utilizes a linear slide (also known as a hob slide) which carries the hob and its support spindles, which are typically located at both ends of the tool. The hob slide motion is parallel to the longitudinal axis of the cylindrical tool. The drive means required to rotate the tool is connected to one of the two support spindles by means of a sliding splined or keyed shaft in many examples. Alternately, portions of the hob drive train, or even the entire hob drive train and its prime mover, may be incorporated into the hob slide. In the last case the tool may be shifted without the need for a sliding joint in the hob drive train.
In all of these examples, the hob slide is ordinarily mounted upon an angularly adjustable trunnion, hereafter called a hob head, which swivels about an axis perpendicular to and intersecting the hob's central axis. The hob head is rotated only as a machine setup function, then clamped in position to present the hob at an appropriate angle to the workpiece. Examples of typical hob head/hob slide configurations can be found in U.S. Pat. No. 2,906,178 to Cotta, U.S. Pat. No. 3,143,040 to Baumann, and U.S. Pat. No. 3,301,134 to Daniel.
U.S. Pat. No. 4,850,155 to Sulzer discloses hob grinding of helical gears using a threaded grinding tool, by a process of continuous hob shifting combined with linear motion parallel to the workpiece central axis (axial motion) to control feeding of the grinding tool. Such axial motion, often provided by a tool slide upon which the hob head and hob slide are mounted, is a normal feature of general-purpose gear hobbing machines, for the sake of developing the full face width of spur, helical or spline teeth.
In contrast to shifting the tool by means of a hob slide, another method has been accomplished, employing a sliding cylinder or spindle assembly which carries the hob. The assembly replaces a hob slide, but is likewise housed typically in an angularly adjustable hob head, with a sliding drive connection between the shifted assembly and the prime mover. U.S. Pat. No. 2,481,974 to Bradner discloses a telescoping arrangement of a sliding spindle inside a fixed sleeve for shifting the hobbing tool. U.S. Pat. No. 2,537,967 to Carlin discloses a sliding cylinder for hob shifting wherein the hobbing tool can be either continuously or incrementally advanced. U.S. Pat. No. 2,690,701 to Zimmermann et al. discloses manual shifting of a sliding spindle located between sections of a split hob head to effect hob shifting. Another sliding cylinder is shown in U.S. Pat. No. 2,769,375 to Moncrieff wherein the cylinder is utilized for hob shifting in a horizontal hobbing machine.
Another approach to positional movement of the hobbing tool is found in U.S. Pat. No. 2,857,818 to Zimmermann et al., wherein two rotatable trunnions and a translatable carriage located therebetween are utilized to shift and orient a hobbing tool. The method presented by Zimmermann et al. must employ continuous tool shifting over the entire tool length with each cut, and the practical tool length itself is determined by the face width of the gear to be generated. These limitations are not encountered with the more typical constructions previously described.
A hobbing machine utilizing an endless chain cutting tool is set forth in U.S. Pat. No. 4,318,648 to Deprez et al. The links of the chain each represent a tool with each successive link being offset from the preceding tool. The chain, arranged in this offset manner, has the same effect on a rotating gear blank as a cylindrical hob having helically arranged stock removing surfaces. In the instance of the endless chain, the workpiece is shifted periodically to equalize wear on the blades carried by the chain.
However, in the gear producing industry today, the most prevalent type of gear hobbing machines are those employing cylindrical tools, an angularly adjustable hob head, and an accompanying linear slide mechanism superimposed upon the hob head for shifting the hobbing tool along its own longitudinal axis.
In each of the above-described hobbing machines setting forth hob shifting, problems are encountered which are associated with the mechanism that must be included in order to position the hobbing tool. One problem encountered on machines of the prior art involves the space that must be reserved on the hob head to accomplish substantial shifting of the tool and its accompanying slide. While long tools are desirable to reduce the frequency of exchange, the size of the hob head needed to support substantial tool shifts, and to adequately clamp the translated mass, in practice limits the amount of tool shift that is workable, particularly on machines with compact floor space requirements. Prior art machines offering large shift ranges feature hob heads with pronounced, imbalanced radial extensions to provide adequate space for both the required drive train and the reserve space for shifting. Those machines which combine the drive means with the hob slide shift a corresponding greater mass in a radially-imbalanced fashion. The pronounced radial imbalance of such hob heads produces a large torsional moment which must be overcome during hob head rotation, and a center of mass which moves over a large range with respect to the hob head support structures. More importantly, such machines in the prior art often limit the extent of tool shifting to an undesirable degree in order to insure that the shifted mass can be clamped securely in the hob head.
On those prior art hobbing machines which orient the work axis vertically, hob heads are generally designed to rotate through an arc of about plus or minus (.+-.) 45 degrees, measured from a horizontal tool position, in order to present the tool correctly for machining gears with teeth of various right or left-handed helix angles. On prior art hobbing machine which orient the work axis horizontally, hob head rotation is similarly limited, measured from a vertical tool position.
Hobs are ordinarily exchanged from the side of the hob head with the lesser radial extension, to limit the required reach. Because of the pronounced radial asymmetry of hob heads equipped with typical tool-shifting slides, it would be impractical to rotate the hob head through 180 or more degrees without producing an unacceptably wide machine envelope. Prior art hobbing machines with the entire drive means incorporated into the hob slide are still less suited to such extensive rotation, due to the interference which would result between the hob drive motor and gearbox elements, and the machine bed and work support structures. Lubrication and hydraulic hosing connecting the hob slide and the hob head, or serving the hob head from remote source points, would also tend to become entangled with other machine elements under conditions of extensive rotation, as would similarly arrayed motor cables.
It would nevertheless be desirable to provide such extensive rotation, in order to oppose the relatively slender hobbing tool outboard support to the relatively robust work support structure under all conditions, in order to reduce the extension of the work support fixture that is typically attached to the machine work spindle. By reducing the extension of the fixture, the work is brought closer to the work spindle support bearings, and the short fixture beam itself is deflected to a lesser extent under the application of heavy cutting forces, which factors contribute to a stiffer, more precise machine.
Because gears of both hands of helix must be machined, in roughly half of the cutting conditions the drive side of the hob head, which is lobed to house the drive gearing and which suffers the greater radial extension, must be rotated downward toward the work support structure and machine bed, on prior art machines with typically limited hob head rotational capacity. Great radial extension thus invites interference, particularly as the helix angle of the workpiece increases. This condition is ordinarily compensated for by lengthening the work fixture to an undesirable degree. Structurally detrimental cutouts in the machine bed or work support structure may even be necessary to provide an acceptable rotational range.
On prior art gear hobbing machines with the work axis oriented vertically, it is also necessary to elevate the hob head more substantially above the machine bed to the extent that the work fixturing is elongated, resulting in proportionately greater moments which must be resisted by the hob head support structure. These moments are in themselves detrimental to machining precision. Certain hobbing machines in the prior art have attempted to reduce work fixture height by combining an exaggerated hob overhang with a tall, slender work spindle housing. Such attempts have not been optimally successful because of the loss of stiffness inherent in such structural tradeoffs, combined with the inability in these constructions to provide extensive hob rotational capacity as described above.
A further difficulty encountered with hobbing machines of the prior art is the increase in tool overhang which results from the thickness of the hob slide itself, resulting in increased detrimental moments upon the hob head support structure. The hob slide itself, and the provisions to clamp it successfully over its range of motion, also substantially increase the overhung mass of the hob head.
An additional problem with prior art hobbing machines is the exposure of hob slideways to processing fluids, chips, and swarf, which can lead to their deterioration, as these slideways are located in a position which is very difficult to protect effectively, in close proximity to the point of machining. The drive means and train that produce hob shifting must also be located on the hob head and therefore within the cutting chamber, thus exposing the associated motor and cables to the same adverse conditions.
It is an object of the present invention to provide a gear hobbing machine of greatly simplified structure wherein the stated drawbacks of the prior art hobbing machines are substantially eliminated.
It is a further object of the present invention to provide a gear hobbing machine capable of shifting the hobbing tool without a hob shifting slide or cylinder.